The rapidly expanding microelectrical mechanical systems (MEMS) market and other areas of semiconductor processing can utilize etching techniques to achieve deep trenches in silicon substrates. For example, some microfluidic devices, chemical, biological and optical transducers can benefit from having deep, high aspect ratio trenches with extremely smooth sidewalls. Dynamic random access memory (DRAM) and/or complementary metal oxide semiconductor (CMOS) devices, among others, are also applications where deep trench etching can be advantageous.
The Bosch process is one process traditionally used for deep silicon etching, and is carried out by using alternating deposition and etching cycles. Although useful in many applications, traditional Bosch processes result in the formation of “scallops” on sidewalls of etched structures. These “scallops”, which are one type of sidewall roughness, are a direct consequence of the alternating deposition and etching cycles. These scallops detract from the use of Bosch processing in applications where high aspect ratio trenches with extremely smooth sidewalls are desired.
One conventional approach to address the scallops is to include additional gases, such as oxygen or nitrogen, or to use shorter cycles during the Bosch process to encourage more anisotropic etching behavior. Although this can reduce scallops somewhat, the use of additional gases can make etching difficult to control and can reduce etching rates (e.g., decrease overall process throughput). In particular, an initial gas surge or flow burst in each cycle can affect process reproducibility and stability. Efforts to reduce the initial gas surge by maintaining a minimal flow rate during the “off” cycle can cause process flow gas overlap and may result in long cycles (e.g., low process throughput). Therefore, conventional techniques for limiting sidewall scallops in the Bosch process are less than ideal.